1. Field of the Invention
The present invention relates to the field of optical communications. More particularly, the present invention relates to the field of avalanche photodiodes.
2. Related Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand occurs within and between metropolitan areas as well as within communications networks. Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand, together with the growing number of distributed computing resources, has resulted in a rapid expansion of the number of fiber optic systems required.
Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities. Other advantages of using light signals for data transmission include their resistance to electro-magnetic radiation that interferes with electrical signals; fiber optic cables' ability to prevent light signals from escaping, as can occur electrical signals in wire-based systems; and light signals' ability to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire.
In a typical fiber-optic network, the transmission and reception of data is not strictly limited to optical signals, however. Digital devices such as computers may communicate using both electronic and optical signals. As a result, optical signals need to be converted to electronic signals and electrical signals need to be converted to optical signals. To convert electronic signals to optical signals for transmission on an optical fiber, a transmitting optical subassembly (TOSA) is often used. A TOSA uses an electronic signal to drive a laser diode or light emitting diode to generate an optical signal. When optical signals are converted to electronic signals, a receiving optical subassembly (ROSA) is used. The ROSA has a photodiode that, in conjunction with other circuitry, detects optical signals and converts the optical signals to electronic signals. A transceiver is a common device that incorporates both a TOSA and a ROSA.
Photodiodes have a two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. One common photodiode is the avalanche photodiode (APD). Generally, an APD is a photodiode that internally converts optical signals into electrical signals and then amplifies the electrical signal through avalanche multiplication. APDs are widely recognized as having an increased sensitivity compared to other optical receivers. This is accomplished as a photocurrent gain applied to the devices increases the sensitivity of the device.
In operation, a large reverse-bias voltage is applied across the active region of an APD. For example, a device may have a breakdown voltage between about 40 volts and about 70 volts for InP based APDs, and up to thousands of volts for Si based devices. Each photon from an optical signal impinging the absorber layer generates an electron-hole pair or a carrier, generating an additional current. When the device's reverse-bias voltage nears breakdown level, the hole-electron pairs collide with ions and/or the crystal lattice to create additional hole-electron pairs, thus achieving a signal gain. This voltage causes the electrons/holes initially generated by the incident photons to accelerate as they move through the APD active region. An avalanche layer in the APD is designed such that one carrier causes an avalanche of other carriers where the number of other carriers is dependent on the gain of the APD. To obtain a desired response from an optical signal, the device may be biased only about 5%-10% below the breakdown level so that the device has great sensitivity to optical power.
FIG. 1 illustrates one step in forming a typical APD. While APD structures vary greatly in form and methods of production, FIG. 1 provides a good background for the present discussion of APDs. As depicted, APD 100 includes avalanche layer 102 having a diffusion region 104 therein. Underneath avalanche layer 102 is a charge layer 108. Underneath charge layer 108 is absorber layer 110, which in turn is over substrate 112. A bottom electrode 114 and a top electrode (not depicted), which are oppositely charged, apply a voltage across the APD. The charge layer 108 helps moderate the electrical field.
The avalanche layer 102 may be formed of a material such as, for example, InP. The avalanche layer 102 is where the electrons/holes initially generated by the incident photons accelerate and multiply as they move through the APD active region. The diffusion region 104 is formed in the center region of avalanche layer 102 with an implanted dopant material, for example zinc, to form, for example, a p+ InP diffusion region 104. As depicted by mask 106, the diffused area of the diffusion region is a direct result of the position of the mask 106. The absorber layer 110 is formed on a substrate 112. As the name implies, the absorber layer is where an optical signal is absorbed.
The process of fabricating InP/InGaAs photodiodes, and APDs in particular, involve at least one diffusion step to form diffusion region 104, and other subsequent steps to control edge gain. Edge gain results from the fact that the electric field is higher at the edges of an APD active region, which roughly has slightly less depth than at the center. Illustrated in FIG. 2 is a cross section of the detected signal strength in an APD. The illustrated peaks correspond to the signal strength at the edge of the APD while the encompassed trough indicates the signal strength at the center of the APD. It can be clearly seen how the signal strength at the edges of the APD is much higher than at the center. This is because the outer region of the detector has a higher responsivity, or ratio of current output to light input, than the center region. By the time the gain in the center achieves an optimal value, the gain at the edges is much higher and causes high level of dark current and edge breakdown limiting the device performance. The edge breakdown is a phenomenon known also as an edge effect.
Additionally, edge effect results from the fact that detectors only provide fast response in their center region despite the higher responsivity at the edges. Thus, the response time, or the time needed for the photodiode to respond to optical inputs and produce and external current, can be affected by edge effect if the APD is focused on the edge rather than the center. This can be a large issue since high edge responsivity can cause problems when aligning an optical fiber to the detector. For example, the higher responsivity on the edge can mislead a user into thinking they have aligned the fiber to the center region when they have actually improperly aligned the fiber to the APD. Because response time is much slower at the edge, however, this misalignment will reduce the response time of the detector. In addition, a misaligned APD will primarily receive impingent photons from an optical fiber on the edge, rather than the center. This is particularly disadvantageous because the center has faster response time, the edge has greater problems with noise, and not all of an optical signal launched by an optical fiber may reach the corresponding APD.
Conventional methods of forming APDs have taken various approaches to controlling the edge effect. For example, some conventional methods use double diffusion. Such methods include forming a first wide mask and then doping. Those skilled in the art will appreciate that “doping” involves the addition of a particular type of impurity in order to achieve a desired n-conductivity or p-conductivity. The first mask is removed and a second, narrower mask is deposited and a deeper doping is performed. This method controls edge effect by creating a thinner diffusion region at the edge, increasing the distance from the diffusion region at the edge to the underlying charge layer, thus reducing the responsivity. Another conventional method known in the art for controlling the edge effect is the etching of curved surfaces prior to diffusion.
Each of these methods, however, as well as others known in the art but not mentioned herein, requires multiple steps to form a diffusion region. Such additional steps raise the cost of forming avalanche photodiodes. In addition, such complicated methods are difficult to control in fabrication processes, often resulting in a low yield.
Accordingly, it would represent an advance in the art to provide a more straightforward method with fewer steps to control the edge effect.